The aim of this project is the elucidation of the mechanism of spontaneous rhythmical activity, especially in cardiac muscle. This goal will be approached by the investigation of the mode of action of drugs which induce or suppress rhythmical activity in molluscan hearts, particularly the heart of the mussel, Modiolus demissus. Both 5-hydroxytryptamine (5HT) and lanthanum depress the pacemaker of isolated Modiolus hearts. The working hypothesis, is that both act by occupying superficial calcium binding sites on the cell membrane; this idea will be tested. The effects of 5HT and lanthanum on the electrical and mechanical properties of the myocardium will be investigated by means of single and double sucrose gap techniques. Membrane and action potentials, and membrane resistances will be monitored. The effect of 5HT and lanthanum on calcium distribution in the cell will be studied and compared by the use of classic Ca 45 ions uptake and washout experiments. Peak C is a substance extracted from molluscan nervous systems. It induces rhythmical activity in quiescent molluscan hearts, and augments the beat in those which are active. Peak C is still impure. It will be purified, identified, and the structure proven by synthesis. The oscillogenic activity of Peak C will be studied using the sucrose gap and isotopic calcium techniques.